The present invention relates to a method and apparatus for generation and collection of ultra fine particles which are used in order to obtain organic photo-conductive ultra fine particles used in the production of an electrophotographic photoreceptor, and more particularly relates to a method and apparatus by which ultra fine particles are generated and recovered by the gaseous vaporization method.
An electrophotographic photoreceptor is basically composed of an electrically conductive base and a photosensitive layer provided thereon. Selenium is conventionally used for a photo-conductive material to form the photosensitive layer. Cadmium sulfide and zinc oxide are used as an inorganic photo-conductive material.
Recently, attempts have been made in order to improve film forming property by utilizing an organic photo-conductive material and to improve productivity by adopting a coating method. Organic photo-conductive materials are advantageous in that: color sensitivity can be freely controlled by selecting an appropriate sensitizer for a dye or a pigment to be used. The following compounds have been known as an organic photo-conductive material: poly-N-vinylcarbazole, and 2,5-bis (P-diethyl aminophenyl)-1,3,4-oxadiazole.
On the other hand, ultra fine particles, the particle size of which is defined by .mu.m or .ANG., have been investigated recently. These ultra fine particles are characterized in that: they exhibit high activity as the specific surface area increases.
The following methods are generally used to obtain ultra fine particles or to adhere the ultra fine particles onto the surface of the base material: a gaseous phase generation method (such as an electric furnace method, a chemical flame method, a plasma method, or a gaseous vaporization method), a liquid phase method (such as a precipitation method or a solvent vaporization method), or a physical crushing method (such as a sand mill, or a ball mill).
In general, when a material is vaporized in a high vacuum state and deposited onto the surface of a base material, a vapor deposited film is formed, so that the vaporized material can not be recovered in the form of particles. However, in the case of the aforementioned gaseous vaporization method, it is possible to collect the vaporized material in the form of ultra fine particles, so that this method has been watched with keen interest. By the gaseous vaporization method, ultra fine particles are recovered in such a manner that: an inert gas is charged into a vacuum vessel, the pressure of which is maintained at 10.sup.-2 to 10.sup.2 Torr; in an atmosphere of the inert gas, various materials are heated so that they can be vaporized or sublimated; the obtained vaporized molecules collide with the inert gas molecules, then gradually cool and cohere with one another so that ultra ultra fine particle are formed; and the formed ultra ultra fine particle are recovered.
Conventionally, the aforementioned gaseous vaporization method has been mainly studied to obtain inorganic or metallic ultra fine particles. However, it has been studied in order to obtain organic ultra fine particles as described in "Functional Materials" page 44-49, vol. June 1987.
As illustrated in FIG. 7-a, the first method comprises the following processes: a vessel 50 in which vaporizable material M is held, a plane or curved plate 51 onto which the vaporized material is adhered, and a collection vessel into which the ultra fine particles are recovered, if necessary, are provided inside a vacuum chamber (not illustrated); an inert gas is charged into the vacuum chamber; a vessel 50 or vaporizable material M is heated so that material M can be vaporized; the vaporized material adheres onto the aforementioned curved plate 51; and after that, the material deposited on the curved plate 51 is scraped off by a brush 53 as illustrated in FIG. 7-b so that the ultra fine particles are recovered into a collection box 54.
As illustrated in FIG. 8 and FIG. 9-a, the second method comprises the following processes: while a plastic web 60 onto which the material is deposited, is unreeled from a reel 61, the plastic web 60 is wound by a reel 62; while the aforementioned process is conducted, vaporizable material M is vapor-deposited onto the surface of the web 60 in the same manner as the first method; when the material is recovered, the web 60 is unreeled by the reel 62 as illustrated in FIG. 9-b; and in the process described above, material M deposited on the surface of the plastic web is scraped off by a brush or the like so that the material can be recovered into the collection box 54.
In the case of the conventional method described above, there is no scatter-preventing-wall, which restricts the movement of vaporizable material M, in the space between the vessel 50 for vaporizable material M and the curved plate 51 or the web 60.
However, in the case where vaporized material M is freely scattered and deposited in the conventional manner described above, some part of material M is scattered to the portion other than the surface of a body onto which material M must be deposited. The temperature of a vacuum chamber wall and that of the members disposed inside the wall, are lower than the temperature of the vaporizing source, so that the scattered material thermally deposits on the inner wall of the vacuum chamber or the surfaces of the members provided inside the chamber. The material deposited in the manner described above can not be recovered, so that the ratio of collection is not good. The ultra fine particles deposited on the inner wall of the vacuum chamber or the surfaces of the inner members, must be cleaned and removed before the production of the successive batch is started, which requires much labor. Accordingly, the productivity can not be improved.
It is a primary object of the present invention to prevent the collection ratio of ultra fine particles from deteriorating so that the collection ratio can be improved, and to reduce the labor required to clean the inner wall of the vacuum chamber.
From the viewpoint of electrophotographic characteristics, it is desirable that the grain size of a photosensitive material is reduced. An obtained grain size is influenced by the pressure, vaporizing speed, and distance from the vaporizing source to the surface onto which the material must be deposited. When these factors are set to appropriate values, the grain size can be reduced. However, there is a limit in the method described above, so that the grain size can not be reduced further.
It is a secondary object of the present invention to provide a generation and collection method by which ultra fine particles of smaller grain size can be obtained through the gaseous vaporization method.